The value of tall fescue [Lolium arundinaceum, Schreb. S. J. Darbyshire] in terms of productivity and persistence in agro-ecosystems arises in part from association with Neotyphodium coenophialum (Morgan-Jones and Gams, Glenn, Bacon, and Hanlin) endophyte. The influence of novel, non-ergogenic endophytes on photosynthate partitioning and nutritive value of tall fescue is unclear, especially when simultaneous stresses (e.g., defoliation and shading) are imposed on the association. We conducted a field experiment, in shaded and open sites, using tall fescue (cultivar Jesup) plants that were host to either a native or a novel non-ergogenic fungal endophyte (MaxQ™), or that were devoid of endophyte.  

Results/Conclusions

Our data suggest that carbon isotope discrimination (Δ) is an indicator of response to light, since Δ increased as the relative amount of shading from co-occurring trees increased. This trend was observed throughout the growing season and was consistent whether Δ was expressed relative to potential evapotranspiration (ET_o), leaf dry matter content (LDMC) or the nutritive value index of herbage energy relative to herbage crude protein (TDN:CP). Carbon isotope discrimination decreased as photosynthetic nitrogen use efficiency (PNUE) increased, with the response associated with microsite. The trend we observed correlated to multiple simultaneous stresses, including defoliation, and agrees with previous observations on Δ response to light gradients. The discrimination index might be useful for identifying shade tolerant perennial grasses for use in silvopasture.